Vitamin A is required for a wide variety of physiologic processes, including vision, differentiation, morphogenesis and growth. Until very recently these tasks were thought to be performed by two metabolic products of vitamin A, retinaldehyde as the chromophore in the eye, and retinoic acid as a ligand for a variety of nuclear receptors concerned with transcription. The list of mediators needs to be amended to reflect at least two additional mediators discovered in the mentor's laboratory. These are the retro-retinoids 14-hydroxy-retro-retinol (14HRR) and anhydro-retinol (AR). 14HRR appears to be intimately involved in growth control, as every growing cell examined so far from drosophila to man synthesizes this compound from retinol. While many normal or transformed cell types can grow in the absence of retinol, others , notably hemopoietic cells, are critically dependent on retinol or the metabolite 14HRR. They die without it. In contrast to 14HRR that can be considered as a growth agonist, AR has strikingly opposite effects on cells: it causes growth arrest, most likely by displacing 14HRR from a common receptor.